1. Field of the Invention
The present invention relates to semiconductor integrated circuits, and more particularly to integrated circuit inductors that are magnetically-coupled for the purpose of creating a high frequency transformer.
2. Prior Art
In wireless communications, an antenna is commonly coupled, typically by means of passive components, to a transformer. In many cases, a balanced-to-unbalanced (BALUN) transformer is used. Such a transformer allows the conversion of a single-ended signal into a differential signal and vice versa. In wireless communications, the antenna receives a single-ended radio frequency (RF) signal. The signal is converted to a differential signal using a BALUN transformer. The operation of BALUN transformers is well-known in the art, and such transformers are usually represented by the schematic 10 shown in FIG. 1A. The unbalanced side of the BALUN transformer has two ends marked 12 and 14 respectively. The balanced side of the BALUN transformer has three connections, two on each end of the inductor marked 22 and 26 respectively, and one at the center of the inductor, marked 24. The balanced side provides for a differential mode. In some cases the inductors of the transformer are wound around a core, directly impacting the mutual inductance between the primary and secondary inductors and therefore the performance of the transformer. Typically node 14 of the primary inductor and node 24 of the secondary inductor are AC grounded, as shown in FIG. 1B.
With the advent of solid state electronics, the ability to integrate components in a single semiconductor device has increased manyfold. This allows the reduction in size, power consumption, and cost, and further provides overall improvement in system performance. It is therefore natural that many attempts have been made to integrate transformers, including BALUN transformers, in order to take advantage of these features. Providing a symmetrical BALUN transformer has been known to be a challenge in the art, as specifically shown in U.S. Pat. No. 6,608,364 by Carpentier (hereinafter “Carpentier”) and U.S. Pat. No. 6,707,367 by Castaneda al. (hereinafter “Castaneda”). Carpentier suggests an implementation of a BALUN transformer that has five metallization layers, therefore requiring a complex manufacturing process having many layers often restricting conductor routing over the BALUN transformer. Castaneda suggests an elaborate scheme to provide a symmetrical BALUN transformer, also requiring several layers of metal and dielectric as shown in the various Figs. of Castaneda. Another example may be found in U.S. Pat. No. 6,882,263 by Yang et al. Symmetrical primary and secondary windings of an on-chip BALUN transformer are shown. However, the issue of capacitive coupling between the primary and secondary windings is not addressed, as the windings are essentially positioned such that a maximum capacitive coupling is achieved, having a disadvantage in operation at high frequencies, for example several GHz, as the capacitive coupling will tend to short-circuit the BALUN at these higher frequencies.
As the demand for integrated circuit radios increases, many attempts have been made to integrate transformers and/or transformer BALUNs onto radio frequency integrated circuits. However, such integration has been limited due to flux leakage, capacitive coupling limits, and significant series resistance. To reduce these limitations, advances have been made in transformer IC design including coplanar interleaved transformers, toroidal and concentric transformers, overlay transformers and symmetric coplanar transformers. Coplanar interleaved transformers have the primary and secondary windings interleaved on the same integrated circuit layer, where the primary and secondary windings are constructed of planer metal traces. While coplanar interleaved transformers reduce size and are widely used, they suffer from low quality (Q) factor, small coupling coefficients, and, if used as a BALUN, the center tap is often at an undesirable location, resulting in an asymmetric geometry. As is known, asymmetry of a transformer winding causes an imbalance in the resulting differential signal and/or an imbalance in the resulting single ended signal from a differential signal.
There is therefore a need in the art for a BALUN transformer which is essentially symmetrical, can be implemented in a minimal number of layers of metal, and still provide the electrical characteristics of a BALUN transformer, and especially a reduced capacitive coupling, for the purposes of RF applications, for example in the gigahertz range.